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Datorteknik F1 bild 1
The Big Picture: Where are We Now?
Control
Datapath
Memory
Processor
Input
Output
Control
Datapath
Memory
Processor
Input
Output
Network
Datorteknik F1 bild 2
I/O System Design Issues
Processor
Cache
Memory - I/O Bus
MainMemory
I/OController
Disk Disk
I/OController
I/OController
Graphics Network
interrupts
Performance
Expandability
Resilience in the face of failure
Datorteknik F1 bild 3
I/O Devices
Connected to the Backplane bus– Hard disk controllers
– Graphics adapters
– Serial I/O
– Sound Cards
– Network adapters
– Virtual Reality Helmet Gloves Quake controller
Datorteknik F1 bild 4
I/O Device Examples
Device Behavior Partner Data Rate (KB/sec)
Keyboard Input Human 0.01
Mouse Input Human 0.02
Line Printer Output Human 1.00
Floppy disk Storage Machine 50.00
Laser Printer Output Human 100.00
Optical Disk Storage Machine 500.00
Magnetic Disk Storage Machine 5,000.00
Network-LAN Inp or Outp Machine 20 – 1,000.00
Graph. Display Output Human 30,000.00
Datorteknik F1 bild 5
I/O System Performance
I/O System performance depends on many aspects of the system (“limited by weakest link in the chain”):
– The CPU
– The memory system: Internal and external caches Main Memory
– The underlying interconnection (buses)
– The I/O controller
– The I/O device
– The speed of the I/O software (Operating System)
– The efficiency of the software’s use of the I/O devices
Datorteknik F1 bild 6
I/O Performance
I/O Bandwidth– How much data can we move from A to B/time unit
– How many I/O operations can we perform/time unit
Response time– The total time to perform a task
Latency per access Bandwidth
Datorteknik F1 bild 7
Simple Producer-Server Model
Throughput:– The number of tasks completed by the server in unit time
– In order to get the highest possible throughput: The server should never be idle The queue should never be empty
Response time:– Begins when a task is placed in the queue
– Ends when it is completed by the server
– In order to minimize the response time: The queue should be empty The server will be idle
Producer ServerQueue
Datorteknik F1 bild 8
Throughput versus Response Time
20% 40% 60% 80% 100%
ResponseTime (ms)
100
200
300
Percentage of maximum throughput
Datorteknik F1 bild 9
Throughput Enhancement
In general throughput can be improved by:– Throwing more hardware at the problem
– reduces load-related latency
Response time is much harder to reduce:– Ultimately it is limited by the speed of light (but we’re far from it)
Producer
ServerQueue
QueueServer
Datorteknik F1 bild 10
Magnetic Disk Purpose:
– Long term, nonvolatile storage
– Large, inexpensive, and slow
– Lowest level in the memory hierarchy
Two major types:– Floppy disk
– Hard disk
Both types of disks:– Rely on a rotating platter coated with a magnetic surface
– Use a moveable read/write head to access the disk
Advantages of hard disks over floppy disks:– Platters are more rigid ( metal or glass) so they can be larger
– Higher density because it can be controlled more precisely
– Higher data rate because it spins faster
– Can incorporate more than one platter
Registers
Cach
e
Mem
ory
Disk
Datorteknik F1 bild 11
Hard disk
2-20 Platters 3600-10000 RPM 1-10 Inch Diameter 500-2000 tracks/surface 32-128 sectors/track Cylinder
– All tracks at one position
SectorsTrack
1-10 inches
2-20
Datorteknik F1 bild 12
Hard Disk Average Seek Time (average time to move to a track)
– 8-12 ms
– Does not consider locality actual average seek time may be 25% to 33% lower.
– (Sum of the time for all possible seek) / (total # of possible seeks)
Rotational Latency– 4-8 ms
– Often dominates over Seek Time
– Start reading to ring-buffer when track reached
Transfer Rate– 2-100 Mb/sec
1 2 3 …30 31 32
Datorteknik F1 bild 13
Other Properties
Storage Size 500Mb-160Gb Cylinder 0, Boot Block Partition Information
– Usually a Physical Disk is devided into Smaller Partitions
– File System Information
File System– The “data structure” for storing files and folders
– Usually Hierarchical Folders, sub folders and files File types, (program/data-format) and protection bits
Datorteknik F1 bild 14
Typical Numbers of a Magnetic Disk
Rotational Latency:– Most disks rotate at 3,600 to 10000 RPM
– Approximately 16 ms to 6 ms per revolution, respectively
– An average latency to the desiredinformation is halfway around the disk: 8 ms at 3600 RPM, 3 ms at 10000 RPM
Transfer Time is a function of :– Transfer size (usually a sector): 1 KB / sector
– Rotation speed: 3600 RPM to 10000 RPM
– Recording density: bits per inch on a track
– Diameter typical diameter ranges from 2.5 to 5.25 in
– Typical values: 2 to 100 MB per second
SectorTrack
Cylinder
HeadPlatter
Datorteknik F1 bild 15
Disk I/O Performance
Disk Access Time = Seek time + Rotational Latency + Transfer time + Controller Time + Queueing Delay
ProcessorQueue
DiskController
Disk
Service RateRequest Rate
Queue
DiskController
Disk
Datorteknik F1 bild 16
Example 512 byte sector, rotate at 5400 RPM, advertised seeks is 12 ms,
transfer rate is 4 MB/sec, controller overhead is 1 ms, queue idle so no service time
Disk Access Time = Seek time + Rotational Latency + Transfer time + Controller Time + Queueing Delay
Disk Access Time = 12 ms + 0.5 / 5400 RPM + 0.5 KB/4 MB/s + 1 ms + 0
Disk Access Time = 12 ms + 0.5 / 90 RPS + 0.125 / 1024 s + 1 ms + 0
Disk Access Time = 12 ms + 5.5 ms + 0.1 ms + 1 ms + 0 ms Disk Access Time = 18.6 ms If real seeks are 1/3 advertised seeks, then its 10.6 ms, with
rotation delay at 50% of the time!
Datorteknik F1 bild 17
I/O Benchmarks for Magnetic Disks
Supercomputer application:– Large-scale scientific problems => large files
– One large read and many small writes to snapshot computation
– Data Rate: MB/second between memory and disk
Transaction processing:– Examples: Airline reservations systems and bank ATMs
– Small changes to large sahred software
– I/O Rate: No. disk accesses / second given upper limit for latency
File system:– Measurements of UNIX file systems in an engineering environment:
80% of accesses are to files less than 10 KB 90% of all file accesses are to data with sequential addresses on the disk 67% of the accesses are reads, 27% writes, 6% read-write
– I/O Rate & Latency: No. disk accesses /second and response time
Datorteknik F1 bild 18
SCSI/EIDE I/O Bus
SCSI – General purpose interface for HDs, Scanners, Streamers, etc
– Both synchronous/asynchronous operation
– 160MB/sec (Ultra160)
– 15 Devices/Bus Mastering/Self Selection Arbitration(Wide)
EIDE– HDs and HD like devices (CD-ROMs etc)
– Synchronous
– 100MB/sec (ATA100)
– 2 Devices on each controller
– HD controller on Interface
Datorteknik F1 bild 19
Reliability and Availability
Two terms that are often confused:– Reliability: Is anything broken?
– Availability: Is the system still available to the user?
Availability can be improved by adding hardware:– Example: adding ECC (Error Correcting Code) on memory
Reliability can only be improved by:– Bettering environmental conditions
– Building more reliable components
– Building with fewer components Improve availability may come at the cost of lower reliability
Datorteknik F1 bild 20
Disk Arrays
A new organization of disk storage:– Arrays of small and inexpensive disks
– Increase potential throughput by having many disk drives: Data is spread over multiple disk Multiple accesses are made to several disks
Reliability is lower than a single disk:– But availability can be improved by adding redundant disks (RAID):
Lost information can be reconstructed from redundant information
– MTTR: mean time to repair is in the order of hours
– MTTF: mean time to failure of disks is tens of years
Datorteknik F1 bild 21
Optical Disks, (CD, DVD)
Disadvantage:– It is primarily read-only media
Advantages of Optical Disks:– It is removable
– It is inexpensive to manufacture
– Have the potential to compete with newtape technologies for archival storage
Datorteknik F1 bild 22
RAID RAID-0
data is split across drives. (striping) higher data throughput. no redundant information is stored.
RAID-1 redundancy by writing all data to two or more drives (mirroring). faster on reads and slower on writes compared to a single drive. no data is lost on disk failure
RAID-2, uses Hamming error correction codes, RAID-3 stripes data at a byte level across several
drives, with parity stored on one drive. RAID-4 stripes data at a block level across several
drives, with parity stored on one drive. RAID-5 distributes parity among the drives.
Datorteknik F1 bild 23
Giving Commands to I/O Devices
Two methods are used to address the device:– Special I/O instructions
– Memory-mapped I/O
Special I/O instructions specify:– Both the device number and the command word
Device number: the processor communicates this via aset of wires normally included as part of the I/O bus
Command word: this is usually send on the bus’s data lines
Memory-mapped I/O:– Portions of the address space are assigned to I/O device
– Read and writes to those addresses are interpretedas commands to the I/O devices
– User programs are prevented from issuing I/O operations directly: The I/O address space is protected by the address translation
Datorteknik F1 bild 24
I/O Device Notifying the OS
The OS needs to know when:– The I/O device has completed an operation
– The I/O operation has encountered an error
This can be accomplished in two different ways:– Polling:
The I/O device put information in a status register The OS periodically check the status register
– I/O Interrupt: Whenever an I/O device needs attention from the processor,
it interrupts the processor from what it is currently doing.
Datorteknik F1 bild 25
Polling: Programmed I/O
Advantage: – Simple: the processor is totally in control and
does all the work
Disadvantage:– Polling overhead can consume a lot of CPU time
CPU
IOC
device
Memory
busy wait loopnot an efficient
way to use the CPUunless the device
is very fast!
but checks for I/O completion can bedispersed among
computation intensive code
Is thedata
ready?
readdata
storedata
yesno
done?no
yes
Datorteknik F1 bild 26
Interrupt Driven Data Transfer
Advantage:– User program progress is only halted during actual transfer
Disadvantage, special hardware is needed to:– Cause an interrupt (I/O device)
– Detect an interrupt (processor)
– Save the proper states to resume after the interrupt (processor)
CPU
I/O Ctrl
device
Memory
addsubandornop
readstore...rti
memory
userprogram(1) I/O
interrupt
(2) save PC
(3) interruptservice addr
interruptserviceroutine(4)
:
Datorteknik F1 bild 27
I/O Interrupt
An I/O interrupt is just like the exceptions except:– An I/O interrupt is asynchronous
– Further information needs to be conveyed
An I/O interrupt is asynchronous with respect to instruction execution:
– I/O interrupt is not associated with any instruction
– I/O interrupt does not prevent any instruction from completion You can pick your own convenient point to take an interrupt
I/O interrupt is more complicated than exception:– Needs to convey the identity of the device generating the interrupt
– Interrupt requests can have different urgencies: Interrupt request needs to be prioritized
Datorteknik F1 bild 28
Delegating I/O Responsibility from the CPU: DMA
Direct Memory Access (DMA):– External to the CPU
– Act as a master on the bus
– Transfer blocks of data to or from memory without CPU intervention
CPU
IOC
device
Memory DMAC
CPU sends a starting address, direction, and length count to DMAC. Then issues "start".
DMAC provides handshakesignals for PeripheralController, and MemoryAddresses and handshakesignals for Memory.
Datorteknik F1 bild 29
Delegating I/O Responsibility from
the CPU: IOP
CPU IOP
Mem
D1
D2
Dn
. . .main memory
bus
I/Obus
OP Device Address
target devicewhere cmnds are
IOP looks in memory for commands
OP Addr Cnt Other
whatto do
whereto putdata
howmuch
specialrequests
CPU
IOP
(1) Issuesinstructionto IOP
memory
(2)
(3)
Device to/from memorytransfers are controlledby the IOP directly.
IOP steals memory cycles.
(4) IOP interrupts CPU when done 1
2
Datorteknik F1 bild 30
Responsibilities of the Operating System
The operating system acts as the interface between:– The I/O hardware and the program that requests I/O
Three characteristics of the I/O systems:– The I/O system is shared by multiple program using the processor
– I/O systems often use interrupts (external generated exceptions) to communicate information about I/O operations.
Interrupts must be handled by the OS because they cause a transfer to supervisor mode
– The low-level control of an I/O device is complex: Managing a set of concurrent events The requirements for correct device control are very detailed
Datorteknik F1 bild 31
Operating System Requirements
Provide protection to shared I/O resources– Guarantees that a user’s program can only access the
portions of an I/O device to which the user has rights
Provides abstraction for accessing devices:– Supply routines that handle low-level device operation
Handles the interrupts generated by I/O devices Provide equitable access to the shared I/O resources
– All user programs must have equal access to the I/O resources
Schedule accesses in order to enhance system throughput
Datorteknik F1 bild 32
OS and I/O Systems Communication Requirements
The Operating System must be able to prevent:– The user program from communicating with the I/O device directly
If user programs could perform I/O directly:– Protection to the shared I/O resources could not be provided
Three types of communication are required:– The OS must be able to give commands to the I/O devices
– The I/O device must be able to notify the OS when the I/O device has completed an operation or has encountered an error
– Data must be transferred between memory and an I/O device
Datorteknik F1 bild 33
Networks
RS-232, copper wire 19.2kbit/sec– Serial Point to Point Protocol (ppp)
LAN (Local Area Network), coaxial 100Mbit/sec
– Ethernet 10Mbit/sec
– Package 128-1530 bytes
– Actually a bus with collision detection
Long Haul Network, fiber 1Gbit/sec– ARPANET (US government) became INTERNET
– Packet Switched Networks
Datorteknik F1 bild 34
Network File System
Mounting Devices over the Network (usually LAN) Local and Network devices transparent to User Network Server - Local Client (a protocol) Needs support by the OS
– Local TCP stack, handles streams of I/O
– Network Server forwards these streams
– “Samba Server” makes Unix devices available to PCs
Datorteknik F1 bild 35
Multimedia and Latency
How sensitive is your eye / ear to variations in audio / video rate?
How can you ensure constant rate of delivery?
Jitter (latency) bounds vs constant bit rate transfer
Synchronizing audio and video streams– you can tolerate 15-20 ms early to 30-40 ms late
Datorteknik F1 bild 36
Graphics Adapters
Each pixel uses a bit array (1-24bits) 1280*1024*24bits/pixel needs approximately 4MB
Red Green Blue
8 + 8 + 8 =24bits/pixel
Yellow White
Datorteknik F1 bild 37
Graphics Adapter Design
Needs to update the Screen 60-100 times/sec (frames)
At 80Hz*4Mb=320Mb/sec Huge Throughput We use special VRAM (Video RAM)
– Shifts out bits to DAC at this high rate
Usually contain a Graphics Accelerator, which
– Move and Copy Blocks of data in local VRAM– Perform operations, like AND/EXOR (bit mask)– Functions Line, Polygon Fill– “3D” functions like:
Polygon Shading, Texture Mapping etc.
Datorteknik F1 bild 38
Multimedia Bandwidth Requirements
High Quality Video – Digital Data = (30 frames / second) (640 x 480 pels) (24-bit color / pel) =
221 Mbps (75 MB/s)
Reduced Quality Video – Digital Data = (15 frames / second) (320 x 240 pels) (16-bit color / pel) = 18
Mbps (2.2 MB/s)
High Quality Audio – Digital Data = (44,100 audio samples / sec) (16-bit audio samples)
– (2 audio channels for stereo) = 1.4 Mbps
Reduced Quality Audio – Digital Data = (11,050 audio samples / sec) (8-bit audio samples) (1 audio
channel for monaural) = 0.1 Mbps
compression changes the whole story!
Datorteknik F1 bild 39
Video Application Example
A system for real-time video recording/playback
– A Frame Grabber, records video to HD
– A Graphics Adapter displays video from HD
640*480*8bits/pixel (256 colors)– 300kb/frame recording or playback
We do NOT want CPU in data path
Datorteknik F1 bild 40
Approach 1 The Frame Grabber records one Frame
(300kb) to local buffer– Frame Grabber gives interrupt
The OS sets up a DMA transfer from FG to RAM
– The DMA gives interrupt
The OS sets up a DMA transfer to HD– The DMA gives interrupt
– The HD gives interrupt, data written to disk
At 25 frames/sec (TV quality) this gives– 2 (first to RAM then to HD) * 300 * 25 = 15Mb/sec
Hmm, no Good! A lot bus activity, too high HD throughput(and this is for just recording)
Datorteknik F1 bild 41
Approach 2
Put MPEG-2 hardware compression on the Frame Grabber, now only 30kb/frame
– Frame Grabber gives interrupt
The OS sets up a DMA transfer from FG to HD– The DMA gives interrupt– The HD gives interrupt, data written to disk
At 25 frames/sec (still TV quality) this gives– 30 * 25 = 750kb/sec
Simultaneous record/playback gives– 1.5 Mb HD throughput, which is possible but still quite
high– A lot of activity on the SCSI bus
Datorteknik F1 bild 42
Approach 3 Frame Grabber
– MPEG-2 hardware compression, now only 30kb/frame
– Graphics Adapter, that can display MPEG-2 frames
– SCSI bus to local HD for video storage
– Interrupt each frame recorded, or finished playback sequence
Best solution! Almost no bus PC bus activity
You get what you pay for, this one will cost you $$$$
Datorteknik F1 bild 43
High Fidelity Audio PCM 44.1 kHz 16bits Stereo (WAV/AIFF) 96 db signal/noice ratio
Left
Right
16 bit signed integer
16 bit signed integer
Datorteknik F1 bild 44
Sound Cards
Wave-Table playback– 16 bit (Stereo at 44.1 kHz)
– 32 voices
– 5.6 Mb/sec (quite high bandwidth)
WaveTableROM/RAM
Signal Processor
MIX
...DA/Filter
Audio
Datorteknik F1 bild 45
Audio I/O In/Out
– Data Buffers
– DMA Channel
– IRQ (Interrupt number) 2 IRQ for full duplex operation
350kb/sec throughput on bus, OK
In Buffer
DSP
DA/Filter Audio OutOut Buffer
Filter/Ad Audio In
Datorteknik F1 bild 46
P1394 High-Speed Serial Bus (firewire)
a digital interface – there is no need to convert digital data into analog and tolerate a loss of data integrity,
physically small - the thin serial cable can replace larger and more expensive interfaces,
easy to use - no need for terminators, device IDs, or elaborate setup,
hot pluggable - users can add or remove 1394 devices with the bus active,
inexpensive - priced for consumer products, scalable architecture - may mix 100, 200, and 400 Mbps
devices on a bus, flexible topology - support of daisy chaining and branching for
true peer-to-peer communication, fast - even multimedia data can be guaranteed its bandwidth
for just-in-time delivery, and non-proprietary mixed asynchronous and isochornous traffic
Datorteknik F1 bild 47
Firewire Operations
Fixed frame is divided into preallocated CBR slots + best effort asycnhronous slot
Each slot has packet containing “ID” command and data Example: digital video camera can expect to send one 64
byte packet every 125 µs– 80 * 1024 * 64 = 5MB/s
isochronouschannel #1time slot
Packet Frame = 125 µsecs
isochronouschannel #1time slot
Time slot available for asynchronous transport
Timing indicator
Datorteknik F1 bild 48
Summary:
I/O performance is limited by weakest link in chain between OS and device
Disk I/O Benchmarks: I/O rate vs. Data rate vs. latency Three Components of Disk Access Time:
– Seek Time: advertised to be 8 to 12 ms. May be lower in real life.– Rotational Latency: 4.1 ms at 7200 RPM and 8.3 ms at 3600 RPM– Transfer Time: 2 to 12 MB per second
I/O device notifying the operating system:– Polling: it can waste a lot of processor time– I/O interrupt: similar to exception except it is asynchronous
Delegating I/O responsibility from the CPU: DMA, or even IOP
wide range of devices– multimedia and high speed networking pose important challenges